This invention relates in general to the vapor deposition of materials on substrates and in particular to a new and useful method and apparatus for the reactive vapor deposition of layers of oxides, nitrides, oxynitrides and carbides on a substrate.
The present invention relates particularly to a method for the reactive vapor deposition of layers of oxides, nitrides, oxynitrides and carbides on supports with simultaneous partial ionization of the vapor and acceleration of the produced ions towards the surface to be coated by an electric field maintained between a vapor source as anode and the supports present at negative potential relative to the anode.
From U.S. Pat. No. 3,562,141 an arrangement is known where the material contained in a crucible is vaporized by an electron beam and the vapor is condensed on the substrates to be coated, the substrate being maintained during the coating at a negative potential of up to -500 V relative to the wall of the vapor deposition chamber. In this arrangement, a so-called low-voltage arc discharge with current strengths of up to 1000 A is maintained furthermore between a hollow cathode and the vaporized material, whereas the current flowing over the substrates could be up to 500 A. To lead away a current of this magnitude, the applied layer had to have the corresponding electric conductivity, of course. Swiss Pat. No. 645,137 discloses a method, for the performance of which a similar low-voltage arc arrangement is used, but additional vaporization power can be supplied to the material to be vaporized by means of an electron beam gun; reference is made to the possibility of keeping the substrates to be coated insulated and placing the amount itself on a potential of for example -500 V which is negative in comparison with the arc plasma. With this known method it is possible, surprisingly, to vaporize at a high vaporization rate practically all materials, i.e. also extremely temperature stable metals and dielectric materials, and to achieve also a high activation of the vapor and of any gases that may still be present in the vaporization chamber or may be introduced therein, e.g. for carrying out a reactive vapor deposition.
With a kinetic energy of the electrons of more than 1 KeV the electron beam brings about the high vaporization rate, and this also when materials of low electric conductivity are being vaporized, and the large number of low energy electrons in the low-voltage arc discharge being about an intensive activation of the vapor or of the supplied reaction gas.
In addition, this method offers the advantage that the coupling, inevitable in other methods of vaporization by means of low-voltage arcs, of process parameters such as vaporization rate, residual gas pressure, residual gas composition, ionization density, etc. can be avoided, so that optimum adaptation to the requirements of the particular application is possible.
A still persisting disadvantage of the last described method is that the application of insulating layers is feasible only to a certain maximum thickness, because, even if higher voltages (500 V) are applied to the substrates, it is no longer possible to remove the charges applied on the surface of the substrates by bombardment with electrically charged ions of the coating material.
Similar problems occur also when materials of good electric conductivity are applied on insulating supports if the electric conductivity of the support is so low that it is no longer able to remove a sufficient quantity of charge from the layers.
The attempt has been made to avoid the problems due to surface charges by providing a reticular metal electrode in front of the insulating surface to be coated. However, this solution is not satisfactory either, because such a reticular electrode causes vapor shadows on the substrates to be coated, resulting in an irregular coating, and because it weakens the intended effect of an ion-supported vapor deposition (ion plating).
When using very fine-meshed nets there is danger also of the net openings being clogged by the vaporized material.
Another known possibility of alleviating the problem of the surface charges in the application of insulating layers is to use high frequency for atomizing the material to be deposited, instead of vaporizing it as usual. In such a high frequency discharge chamber, in fact, the electrode of the smaller surface always takes on a negative charge, i.e. the substrate carrying electrode becomes negative relative to the coating chamber walls of the larger surface. Here the attempt is made to achieve a charge equilibrium in such a way that the positive charge of the substrates supplied by ions of the coating material is compensated by the much greater mobility of the electrons in plasma. If this equilibrium can be maintained during the coating without the substrates being charged to too high a negative potential, uniform coating may be expected. On the other hand, substrate surfaces charged to higher potentials may be harmed due to stronger electric discharges then occurring.
Experience has shown that the above described difficulties arise especially when supports are coated with layers of oxides, nitrides, oxynitrides and carbides.